In preschool children, detection of maladies of the eyes that cause amblyopia, or strabismus (a deviation of the optical axis of one eye from the optical axis of the other eye), is particularly important. As an untreated child becomes older, neural development in the brain tends to permanently suppress vision in the diseased eye. This prevents proper binocular vision and is irreversible. In order to correct this condition, treatment must begin in the early years of life to be effective. As about five percent of children are either born with or develop strabismus or amblyopia, diagnosis and treatment of this condition at as early an age as possible, preferably prior to about 12 months, is highly desirable. However, considerable difficulty may be encountered in properly administering an ocular screening test to pre-verbal children of such a young age.
Additionally, other conditions of the eyes relating to errors of refraction of the transparent media of the eye, such as nearsightedness (myopia) and farsightedness (hypermetropia), and conditions relating to opacities of the eyes, such as cataracts, scars on the cornea, and tumors, as well as problems such as detachment of the retina, need to be diagnosed and treated so as to provide higher quality vision for the patient.
Screening devices for screening a large number of subjects over a relatively short period of time have been proposed for detecting strabismus and other abnormal conditions of the eyes. These devices function by directing light from a light source to the eyes of a subject, which are fixed and focused at a point defined by a fixation light mounted about one degree off-axis with respect to the light source. Generally, a camera utilizing photographic film is positioned as close as possible to the fixation light. As such, off-axis light from the light source enters the eyes and is refracted by the transparent media, which includes the lens, of each of the eyes.
In an individual with normal eyes, an incoming image of the light source is refracted and perfectly focused as a tiny image upon the retina. As the light is off-axis with respect to an optical axis of the eyes, the tiny image of the light source falls generally on the macula, a pigmented structure of the retina, and the fovea, which is coincident with the optical axis of the eyes. A portion of the light is reflected, generating a retinal reflection, or reflex. This retinal reflection is re-refracted by the lens as it exits the eyes and is collimated, thereby to be directly coincident with respect to light from the light source.
In the individual with normal eyes, the camera records a small, well-defined point of light in the center of the pupil, this point being a reflection from the cornea, or outer transparent covering of the eye. The reflection from the macula is not seen due to the refracted reflection being perfectly focused and directed back toward the light source; however, faint illumination of the macula is observed around the reflection from the cornea due to scattering of light within the eye. This faint illumination of the macula with a small, centered reflection from the cornea is the optical signature of a normal eye.
With the flash positioned above the camera lens and separated from the lens and fixation light by about one degree of angular separation, a nearsighted eye (myopia) refracts the outgoing reflection to be diverging and inverted, as opposed to being collimated. If the angle of divergence of the retinal reflection from the myopic eye is large enough, then a portion of the retinal reflection is intercepted by the camera lens. This results in the camera recording a bright crescent from each eye generally in the lower quadrants of the pupil, the crescents appearing in the lower quadrants due to inversion of the retinal reflection effected by the myopic eye. Of course, the distance between the subject and the flash, fixation light, and camera determines sensitivity of the device, with a longer distance allowing greater divergence of the retinal reflection, meaning that a bright crescent will be recorded at lower errors of refraction of the eyes. Conversely, a shorter distance between the subject and the flash, fixation light, and camera allows less divergence of the retinal reflection, causing the camera to record a bright crescent at higher errors of refraction of the eyes. Generally, and irrespective of the sensitivity of the device, the larger the bright crescent, the greater the severity of nearsightedness.
In the instance where the eyes are farsighted (hypermetropia), the retinal reflections diverge as described and are not inverted as they leave the eyes. As such, the bright crescents appear on the same side of the pupil as the flash with respect to the camera lens. With the flash located above the camera as described, the camera records a bright crescent in the upper quadrants of the pupil. Again, the size of the crescent correlates with severity of the farsighted condition.
In the instance where an individual is afflicted by strabismus, a deviation of the optical axis of one eye with respect to the optical axis of the other eye, the point of light reflected by the cornea in the deviating eye is not equidistant from upper and lower and right and left sides of the eye, as compared with the non-deviating eye. Additionally, the retinal reflection or a portion thereof may appear in the deviating eye as a bright source of light from the pupil.
With respect to astigmatism, which is usually associated with nearsightedness or farsightedness, the crescent will be rotated around edges of the pupil in a positive or negative direction a number of degrees corresponding to the particular combination of the axis and optical power of the astigmatic eyes of that individual.
Opacities of the transparent media of the eyes, such as cataracts or scars on the cornea, produce darkened regions in the reflections wherever they are located.
In some of these devices of the prior art, the fixation light is positioned about 18.9 feet from the subject, a distance such that when the eyes are focused on the fixation light, the optical axes of the eyes are essentially parallel and focused on optical infinity. At this distance, slight divergence of the retinal reflection corresponding to about 0.25 diopters refractive error of the eyes is recorded by the camera. However, size of these devices is a drawback; devices of this length are awkward to handle and move and require a table or support of about 18.9 foot length when in use. Further, since these devices are extremely sensitive to refractive errors, children having only minor refractive error defects of 0.25 or 0.5 diopters and who would derive only dubious benefit from corrective lenses were selected during screening as having defective vision.
Finding the length of these longer screening devices to be cumbersome to move and temporarily install, others have proposed reducing size of these devices by shortening the optical path between the camera, light source, and eyes of the subject. However, when eyes of a subject are focused on a relatively near fixation light, the optical axes of the eyes converge on the fixation light, rotating the retinal reflection about the optical axes of the eyes and causing the appearance of an astigmatic condition. Further, normal convergence of the eyes on a relatively near fixation light prevents differentiation of some ocular misalignment disorders.
One of these proposed screening devices is disclosed in U.S. Pat. No. 4,669,836 to Richardson et. al. and includes a foldable base which, unfolded, measures about 2.4 meters in length. At one end of the base is an upwardly extending head positioning station for positioning the head, and thus eyes, of an individual being screened in relatively precise vertical and horizontal planes. At an opposite end of the device is a camera focused in a plane of the head positioning station. An electronic flash unit is mounted below the camera, and a blinking fixation light to draw the gaze of an individual is mounted just above the camera lens.
Problems with this device are, as stated, its length, which, in spite of the fact that it may be folded for storage or transport, requires that it have in excess of 2.4 meters of unobstructed floor space. When folded, its mass is about 22 pounds. Further, since images of subjects eyes are photographically made on film, there is no immediate feedback as to whether eyes of the subject were correctly positioned, if the test was correctly administered, or if the subject was cooperative in looking at the fixation light, a requisite condition for obtaining a satisfactory screening result. Additionally, since the device is less than half the 18.9-foot length required for a subject to focus his/her eyes on infinity, optical aberrations are introduced in the recorded images due to the eyes converging on the fixation light. This impairs accuracy of the device. Further yet, the bright flash may startle very young children.
A screening device similar to that described in U.S. Pat. No. 4,669,836 is described in a paper entitled "The Remote Sensing of Eye Disorders Utilizing the Retinal Reflex Photometer" by S. Hutson Hay and Rhonda Wharry, which was published in Volume 601, pages 107-111 of the 1985 Proceedings of The Society of Photo-Optical Instrumentation Engineers and discloses a device also having a camera and off-axis flash at one end, a fixation light closely proximate the camera, and a head positioning station located 18.9 feet distant at the opposite end of the device. The photographic images may be subjected to limited computer enhancement.
Problems with this device are its length, which requires more than 18.9 feet of unobstructed floor space during use. Additionally, as in the prior device, photographic images are recorded on film which must be developed before the results are obtained. Again, the bright flash may be uncomfortable for small children.
Another paper entitled "Retinal Reflex Photometry as a Screening Device for Amblyopia and Preamblyopic States in Children" by S. Hutson Hay, M.D., Joseph H. Kerr, Robert Rhea Jayrose Jr., Ph.D., James C. White II, and Michael Funke, which was published in the March 1983 edition of the Southern Medical Journal, discloses a hand-held camera with a 1,000-mm telescopic lens, with an off-axis flash mounted below the lens. A light-emitting diode (LED) serves as a fixation light and is mounted just above the lens. The subject's head is carefully stabilized 18.9 feet from the lens; and with the subject gazing at the LED, a photograph with simultaneous flash is taken.
Again, this device requires more than 18.9 feet of unobstructed floor space in order to perform the screening, and photographic images are recorded on film which must be processed in order to obtain a result, and the flash may disturb small children.
U.S. Pat. No. 4,989,968 to Howard L. Freedman discloses a camera having an internal light path of about one meter, the light path being folded by mirrors. The camera utilizes a small, slit aperture and a light source positioned 0.5 mm from the slit aperture. Both the slit aperture and the light source are mounted in rotatable relation with the optical axis of the camera so that they may be rotated through at least a 90-degree arc. A series of LED fixation lights, which may be red or any other color, are mounted just below the slit. Provisions are made to place two photographs on a single sheet of self-developing film. Further, a parallax aiming system is provided which projects two light images onto the forehead of the subject and the distance between the subject and camera adjusted until the light images touch, indicating proper alignment and focusing. In use, two photographs are taken of a single subject, the second photograph taken with the slit aperture and light source rotated 90.degree. from a position used to take the first photograph. This allows observation and recording of two discrete meridians of refractive error juxtapositioned 90.degree. with respect to each other.
Problems with this device are that two photographs are required, increasing the probability that the second photograph will be incorrectly taken due to the subject's vision being disrupted by persistence of the image of the first flash. Additionally, the necessity of taking two photographs with accompanying bright flashes in relatively quick succession may cause problems with small children who, after being startled by the flash during the first photograph, may be somewhat less cooperative during the second photograph. Further, it is necessary for an operator of the device to be actively involved with precise focusing and positioning of the camera and rotation of the slit aperture and flash between photographs, this all making for a relatively complicated and trying procedure, especially when groups of very young children, such as in day care centers, are being screened.
Accordingly, objects of this invention are to provide an ocular disease detection device which is relatively compact, occupying in a longest embodiment only about four feet of space in length and about two feet of space in width, and which during use causes the subject's eyes to become defocused as though the subject were looking at a distant object. Additionally, the device of this invention is uncomplicated in its use. Further, the device does not direct a bright flash of light into eyes of those being tested.